The
Earth Receiver is an E.L.F. (Extremely Low Frequency) receiver is designed
to receive naturally occuring atmospheric radio signals in the 1 to
10 kHz range. This range is not generally used for routine radio communications,
due to the giant antennas and high power levels needed to generate a
useful signal. The lowest allocated radio frequency is 9 kHz. ELF is
defined as those frequencies between 300 and 3000 Hz (0.3 to 3 kHz),
while VLF (very low frequency) is defined as frequencies between 3 and
30 kHz. Below 9 kHz, there are many interesting naturally occuring radio
signals generated by electrical activity in the earth and its atmosphere.
These include sferics, chirps, whistlers, and the “dawn chorus”.
The Earth and surrounding space produce a variety of naturally occurring
radio phenomena from lightning storms, solar events, auroras, geologic
activity, and other related natural events. The most common generator
of natural radio signals are the ever present thunderstorms on our planet.
There are several hundred to more than a thousand thunderstorms occuring
on Earth at any one time. The tremendous energy and the dimensions of
this activity all allow significant RF energy to be generated in the
audio frequency range (30-15000 Hz, or 0.03 to 15 kHz). Since this energy
is electromagnetic, we cannot directly hear it, even though it is in
the audio range. It is often asked by those unfamiliar with electronics,
and physics in general, why these low frequencies, in the audible range,
cannot be heard by the unaided ear. The reason is the mode of propagation.
These waves are electromagnetic in nature, while audible sounds are
mechanical in nature, consisting of changes in pressure in the conducting
medium (water, air, etc.). A number of radio hobbyists have become interested
in this area of radio, and have built or purchased ready made receivers
for this purpose. They have been called “Natural Radio Enthusiasts”,
and there are a few groups on the internet that deal with this field.
You can visit the website of the Long
Wave Club of America, for lots of information on natural radio,
and they have links to many other such sites. If you are interested
in radio, astronomy, space science, weather, or geology you will surely
want to investigate this area of VLF and ELF radio phenomena. And, it
is quite easy to build this receiver kit to listen in on this part of
the radio spectrum. It can be built at a cost equal to that of a pocket
FM stereo receiver /casette player, and is similar in size and weight.
One might think a huge antenna is needed at these low frequencies, but
in practice a small whip antenna will allow atmospherics to be heard,
An active antenna circuit is used for reception, with a receiver input
impedance of more than 10 megohms. This allows the use of a short whip
antenna of 0.3 to 1 meter in length. While no ground is needed, a short
earth spike (a 20 to 35 cm length is adequate, a nail or a knitting
needle works well) connected to the metal case of the receiver with
a 1 to 2 meter length of wire greatly improves reception. Even in rocky
soil areas, it should be easy to insert a short spike into the topsoil.
Since the antenna impedance is so high, several thousand ohms ground
resistance is acceptable. The kit is easy to build and no alignment
or circuit adjustments are required. All components and an NC531
metal case with connectors, switches, hardware, and battery holder
included to build the complete receiver. An optional collapsible
antenna with mating BNC connector(NC1498)
is available. Batteries (4-AA cells or 9V transistor battery) and stereo
headphones not included, use any ordinary portable stereo headphones